This invention is directed to a process for synthesis of alpha, beta-unsaturated acids by reaction between formaldehyde and a carboxylic acid of the formula RCH.sub.2 COOH wherein R is a member of the class consisting of --H, -alkyl, -aryl, -aralkyl, and -cycloalkyl radicals. When R is not hydrogen, the number of carbons in R is preferably from 1 to 18.
This invention is more specifically directed to a process for synthesis of alpha, beta-unsaturated acids, e.g., alpha-methacrylic acid from propionic acid and formaldehyde. The process requires the presence of a catalyst containing a porous silica gel, a Periodic Group IA metal ion such as sodium or cesium ion, and, optionally another material such as tin (IV) oxide. In particular, this invention is directed to a process for maintaining activity and selectivity of such a catalyst in such a process wherein a sufficiently feed soluble or volatile source of alkali metal ions is added during the process whereby the required level of basic metal components of Group IA is maintained upon the catalyst.
Unsaturated acids, such as methacrylic and acrylic acids, acrylonitrile and the esters of such acids, such as methyl alpha-methacrylate, are widely used for the production of corresponding polymers, resins and the like. Various processes and catalysts have been proposed for the conversion of alkanoic acids, such as propionic acid, and formaldehyde to the corresponding unsaturated monocarboxylic acids, e.g., methacrylic acid, by an aldol-type reaction. Generally, the reaction of the acid and formaldehyde takes place in the vapor or gas phase while in the presence of a basic or acidic catalyst.
Various catalysts have been proposed for such reaction. For example, Vitcha, et al., I&EC Product Research and Development, 5, No. 1 (March, 1966) pp. 50-53, propose a vapor-phase reaction of acetic acid and formaldehyde employing catalysts comprising alkali and alkaline earth metal aluminosilicates, silica gel, alumina and the like. U.S. Pat. No. 2,821,543 teaches use of a catalyst comprising a basic metal compound such as a basic salt or oxide upon activated silica. The example taught is 10 wt.% of manganese oxide on activated silica. U.S. Pat. No. 3,051,747 describes the preparation of acrylic acids by reacting an alkanoic acid and formaldehyde in the presence of a catalyst comprising an alkali metal salt of the alkanoic acid supported on alumina. The same reaction is also promoted by catalysts which include alkali metal or alkaline earth metal aluminosilicates, silica gel or alumina. Catalysts of this kind are described in U.S. Pat. No. 3,247,248 which teaches a process for the reaction of formaldehyde and acetic acid or propionic acid in the presence of a natural or synthetic aluminosilicate catalyst that may include alkali or alkaline earth metals, such as the aluminosilicates of sodium, potassium, rubidium, magnesium, calcium, strontium or barium. In addition, the use of silica gel in combination with an alkali metal or alkaline earth metal hydroxide as a catalyst for the reaction is described. U.S. Pat. Nos. 3,840,587 and 3,840,588 teach preparation of alpha, beta-ethylenically unsaturated compounds by vapor-phase reaction of formaldehyde and saturated carboxylic acids in the presence of a Group IA metal compound associated with a silica gel support, the content of the Group IA compound calculated as the hydroxide in the range of 0.01 to 10 weight percent of the completed catalyst. U.S. Pat. No. 3,933,888 teaches the preparation of unsaturated acids, and the preparation of esters and nitriles of such unsaturated acids wherein alkanoic acids, esters of such acids and alkyl nitriles are reacted with formaldehyde in the presence of a basic catalyst comprising pyrogenic silica. Pyrogenic silica is taught as displaying radically different properties from silica gel. The pyrogenic silica is taught as especially effective when treated with activating agents which provide basic sites on the pyrogenic silica catalyst support, such as organic bases, inorganic bases of Groups IA, IIA and IIIB of the Periodic Table, particularly the alkaline metal hydroxides such as potassium hydroxide and cesium hydroxide. The addition of a compound of a metal as an activating agent is taught as increasing the effectiveness of the catalyst.
Other processes and catalysts have been proposed for the preparation of methacrylic acid, esters and nitriles. U.S. Pat. No. 3,089,898 teaches a process and catalyst for preparation of methyl acrylate which comprises contacting vapor mixtures of methyl acetate and formaldehyde with aluminosilicate catalysts, particularly alkaline earth metal zeolites, alkali metal zeolites and zeolites of certain heavy metals such as manganese, cobalt, zinc, cadmium and lead. Aqueous and alcoholic sources of formaldehyde are taught as useful. U.S. Pat. No. 3,089,899 teaches preparation of methyl methacrylate which comprises contacting vapor mixtures of methyl propionate and formaldehyde with zeolite catalysts, particularly certain synthetic zeolites, especially the aluminosilicates of Group IIA of the Periodic Table, such as magnesium, calcium, strontium and barium aluminosilicates, and manganous aluminosilicates. Aqueous or alcoholic formaldehyde or anhydrous paraformaldehyde can be used. U.S. Pat. No. 3,535,371 teaches a process and catalyst for preparing acrylic esters by contacting a gaseous mixture of formaldehyde, an aliphatic carboxylic acid such as acetic, propionic, butyric, valeric, caproic and the like, and an aliphatic alcohol such as methanol, ethanol, propanol, butanol, pentanol, hexanol and the like, in the presence of a niobium oxide catalyst upon a catalyst support of silicon carbide, silica or alumina, but preferably alumina. Formaldehyde used can be of any suitable form, either anhydrous or aqueous. U.S. Pat. No. 4,118,588 teaches a process and catalyst for preparing methacrylic acid and methyl methacrylate which comprises reacting, respectively, propionic acid and methyl propionate with dimethoxymethane in the presence of catalysts based on phosphates and/or silicates of magnesium, calcium, aluminum, zirconium, thorium and/or titanium and in the presence of water. Boric acid and/or urea can also be present. Preferably, the catalysts are modified with alkali metal and/or alkaline earth metal carboxylates and/or alkali metal compounds and/or alkaline earth metal compounds which yield carboxylates under the reaction conditions. Suitable modifiers are the carboxylates, oxides and hydroxides of lithium, sodium, potassium, magnesium and calcium as well as those of beryllium, strontium, rubidium, cesium and barium.
However, the process and catalysts taught heretofore suffer from disadvantages which are greatly minimized in the process of the present invention. The processes as described in Vitcha, I&EC, op. cit. p. 50, are inferior to the present invented process in that conversion of formaldehyde is low when reactant acid concentration is low. Hence, large excesses of acid reactant are used and, thus, acid conversion is necessarily low. Vitcha indicates that as the ratio of reactant acid to formaldehyde decreases, the competitive reaction of formaldehyde with itself to form polymers predominates, to result in lower conversion and yield. U.S. Pat. No. 3,051,747 indicates that yields are low (5 to 11%) and the major product of the process is not an unsaturated compound but a symmetric ketone. Processes described in U.S. Pat. Nos. 3,247,248 and 3,933,888 are also inferior to the process of the present invention. Yield percent based on formaldehyde taught by U.S. Pat. No. 3,247,248 with 5:1 ratios of acid to formaldehyde is between 29 and 40 percent. U.S. Pat. No. 3,933,888 teaches the major products are methacrylic acid and its ester, methyl methacrylate, not methacrylic acid predominantly.
In addition to the above disadvantages, the references fail to appreciate the level of alkali content of the catalyst as being significant. The references also fail to appreciate that there is an apparent loss of alkali content as the reaction continues.
An object of the present invention is to provide a catalyst and process for making alpha, beta-unsaturated acids from formaldehyde and aliphatic carboxylic acids. A further object is to provide a catalyst and process for making alpha, beta-unsaturated acids from formaldehyde and aliphatic carboxylic acids wherein catalyst activity and selectivity is maintained. A further object is to provide a catalyst and process for making alpha-methacrylic acid. Another object is to provide a catalyst and process for acrylic acid. Other objects will appear hereinafter.
Quite unexpectedly it has been found that modified or unmodified silica gel catalysts prepared from precipitated colloidal silica perform in a much superior manner for the present condensation process with respect to conversion and selectivity relative to conventional catalysts, based on silica gel and the like. The improved catalyst formulation has several unexpected results. Whereas conventional catalyst formulations result in low formaldehyde-based yields of methacrylic acid when the ratio of propionic acid to formaldehyde is low, such as 1:1, high yields of alpha-methacrylic acid can be obtained in the process of the instant invention when the ratio of propionic acid to formaldehyde is as low as about 0.1:1. Preferred reactant acid:formaldehyde ratio for the process of the present invention is 1:1 to 5:1, preferably 1:1 or 2:1, with consequent economic advantage. The present invention is preferably with the use of a dry formaldehyde such as gaseous monomer, methanolic formaldehyde, trioxane or paraformaldehyde; and the catalyst preferably contains low levels of the active basic components of Group IA such as sodium or cesium, or other basic components of other groups of the Periodic Table.
Also unexpectedly it has been found that the process of the instant invention wherein an alkali component is added continuously or discontinuously to the feed materials prolongs catalyst activity and selectivity aiding in maintaining product yields. This improvement is shown for a wide variety of alkali-metal-loaded, porous-silica-gel-based condensation catalysts.